1. Field of the Invention
The present invention relates to the ribbon takeup device of an inked ribbon device and to a printer.
2. Description of the Related Art
For a ribbon takeup mechanism used in an inked ribbon device mounted on a small printer, some are known as mechanisms for taking up a ribbon such as a mechanism that uses the tensile force of a tension spring and a mechanism that uses the drive torque of a motor, are known (see Patent Document 1, Patent Document 2).
FIG. 13 is a general diagram showing a mechanism that takes up an inked ribbon by the tensile force of a tension spring.
Referring to FIG. 13, a ribbon takeup mechanism 101 has a pair of ribbon takeup axes 104, each with a ratchet wheel 105, on a ribbon frame 102. Ribbon spools 110, on which a ribbon 116 is wound, are mounted on this pair of ribbon takeup axes 104.
A ribbon feed plate 103 is slidably installed on the ribbon frame 102, with a tension spring 115 between the ribbon frame 102 and the ribbon feed plate 103. A feed claw 106 is provided on the ribbon feed plate 103. The feed claw 106 engages the ratchet wheel 105 through the tensile force of the tension spring 115 to drive the ribbon takeup axes 104. A motor-driven cam 109 is used to stretch the tension spring 115, and the ribbon takeup axes 104 are driven by the tensile force of the stretched tension spring 115 that tends to restore to its original position.
FIG. 14 is a diagram showing how the ribbon takeup mechanism described above drives the ribbon feed plate. Referring to FIG. 14, the tension spring 115 always applies force to the ribbon feed plate 103 into the direction indicated by the arrow. From FIG. 14(a) to FIG. 14(e), the ribbon feed plate 103 moves to the left in the figure as the cam 109 rotates. During this time, the tension spring 115 is stretched. Next, from FIG. 14(f) to FIG. 14(h), the ribbon feed plate 103 moves to the right in the figure by the tensile force of the stretched tension spring 115 that tends to restore to its original position.
When the ribbon feed plate 103 moves to the right as shown in FIG. 14(f) to FIG. 14(h), the feed claw engages the ratchet wheel to rotate the ribbon takeup axes and takes up the ribbon.
FIG. 15 is a general diagram showing the motor-driven mechanism for taking up an inked ribbon.
As with the configuration shown in FIG. 13, a ribbon takeup mechanism 111 shown in FIG. 15 has a pair of ribbon takeup axes 104, each with the ratchet wheel 105, on the ribbon frame 102. The ribbon spools 110, on which a ribbon 116 is wound, are mounted on this pair of ribbon takeup axes 104.
A ribbon feed plate 113 is slidably installed on the ribbon frame 102. The feed claw 106 is provided on the ribbon feed plate 113. As the ribbon feed plate 113 moves, the feed claw 106 engages the ratchet wheel 105 to drive the ribbon takeup axes 104.
The ribbon feed plate 113 has an arm 113a that has a drive mechanism at its end. The drive mechanism comprises a slit 113b formed at the end of the arm 113a and a gear 119 having a column 119a that slides along the opposed sliding surfaces 113c of the slit 113b. When the gear 119 rotates, the column 119a slides along one of the sliding surfaces 113c in the slit 113b to cause the ribbon feed plate 113 to reciprocate linearly.
FIG. 16 is a diagram showing how the ribbon takeup mechanism described above drives the ribbon feed plate. Referring to FIG. 16, the ribbon feed plate 113 is driven by the motor through the gear, the column, and the sliding surfaces. As the gear is rotated by the motor, the ribbon feed plate 113 moves to the left in the figure from FIG. 16(a) to FIG. 16(e) and, after that, to the right from FIG. 16(f) to FIG. 16(h).
When the ribbon feed plate 113 moves to the right in the figure from FIG. 16(f) to FIG. 16(h), the feed claw engages the ratchet wheel to rotate the ribbon takeup axes and takes up the ribbon.
[Patent Document 1] Japanese Patent Laid-Open Publication No. Hei 01-278385
[Patent Document 2] Patent Application No. 3002780
The ribbon takeup mechanism that drives the ribbon takeup axes through a tension spring described above drives the ribbon takeup axes only through the tensile force of the tension spring, meaning that a tension spring must have the tensile force exceeding the load of the ribbon takeup axes. The problem here is that the ribbon takeup axes that have a heavy load require a greater tensile force of the tension spring.
The load of the ribbon takeup axes includes a load generated at a ribbon feed time as well as a load generated when the ribbon feed claw is switched from the ratchet wheel of one ribbon takeup axis to the ratchet wheel of the other ribbon takeup axis when the movement direction of the ribbon is reversed. When the ribbon feed claw is switched through the tensile force of the tension spring, a load heavier than the load at a ribbon feed time is generated. Therefore, a greater tensile force of the tension spring is required and a heavy load is applied to the motor. Another problem is that the motor drive efficiency becomes low because the motor drives the mechanism always under the load of the tensile force of the tension spring.
In FIG. 17(a), the left to the broken line indicates the period of time during which the tension spring is stretched. In this period, the ribbon feed plate is moved to the position where the ribbon feed claw engages the ratchet wheel. The right to the broken line indicates the period of time during which the tension spring is restored to its original position. The ribbon is fed in this spring restoration period by causing the ribbon feed claw to engage the ratchet wheel to rotate the ribbon takeup axes.
To feed the ribbon, the spring torque T1 exceeding the load, required for feeding the ribbon (chain double-dashed line in the figure), is required during the ribbon feed period. To switch the ribbon feed claw from one ratchet wheel to another, the spring torque T2 exceeding the load, required for switching the ribbon feed claw (dashed line in the figure), is required during the switching period.
The above-described ribbon takeup mechanism that uses a motor to drive the ribbon takeup axes solves the problem of the load of the tension spring mechanism that uses a tension spring. However, because a heavy load must be applied to the motor when the ribbon is switched, the motor requires a large driving torque and therefore a large motor is required.
Referring to FIG. 17(b), the left to the broken line indicates the period during which the ribbon feed plate is moved to the position where the ribbon feed claw engages the ratchet wheel, and the right to the broken line indicates the period during which the ribbon is fed. To feed the ribbon, a motor torque exceeding the load, required for ribbon feeding (chain double-dashed line in the figure), is required during the ribbon feed period. To switch the ribbon feed claw from one ratchet wheel to another, a motor torque exceeding the load, required for switching the ribbon feed claw (dashed line in the figure), is required during the switching period.
Normally, the load required for switching the ribbon feed claw is heavier than the load required for feeding the ribbon. Therefore, the peak torque required for the motor is a torque exceeding the load required for switching the ribbon feed claw (dashed line in the figure).